Graphene-Coupled Terahertz Semiconductor Lasers for Enhanced Passive Frequency Comb Operation

Improved comb and dual-comb operation of terahertz quantum cascade lasers utilizing a symmetric thermal dissipation

In the terahertz frequency range, the quantum cascade laser (QCL) is a suitable platform for the frequency comb and dual-comb operation. Improved comb performances have been always much in demand. In this work, by employing a symmetric thermal dissipation scheme, we report an improved frequency comb and dual-comb operation of terahertz QCLs. Two configurations of cold fingers, i.e., type A and B with asymmetric and symmetric thermal dissipation schemes, respectively, are investigated here. A finite-element thermal analysis is carried out to study the parametric effects on the thermal management of the terahertz QCL. The modeling reveals that the symmetric thermal dissipation (type B) results in a more uniform thermal conduction and lower maximum temperature in the active region of the laser, compared to the traditional asymmetric thermal dissipation scheme (type A). To verify the simulation, experiments are further performed by measuring laser performance and comb characteristics of terahertz QCLs emitting around 4.2 THz mounted on type A and type B cold fingers. The experimental results show that the symmetric thermal dissipation approach (type B) is effective for improving the comb and dual-comb operation of terahertz QCLs, which can be further widely adopted for spectroscopy, imaging, and near-field applications.

Graphene-supported tunable bidirectional terahertz metamaterials absorbers

Based on asymmetric graphene ellipses, the tunable propagation characteristics of metamaterial absorber (MMA) have been investigated in the THz region. Two distinct absorption peaks of 84% and 90% are observed at 1.06 THz and 1.67 THz, respectively. Besides a high Q factor exceeding 20, the Fano resonance can also be modulated in a wide range (e.g., the frequency modulation depth reaches more than 43.8% if the Fermi energy level changes in the range of 0.2-1.0 eV). Additionally, a bidirectional THz MMA is achieved by replacing the metal substrate with a uniform graphene layer. If the terahertz wave is incident in the forward direction, the proposed graphene double stripe microstructure shows a typical MMA with its absorption reaching 88%. On the other hand, if the terahertz wave is incident in the reverse direction, the graphene double stripe microstructure behaves as a reflective modulator, and its amplitude and frequency MD will reach 60% and 85%. These results contribute to the design of tunable THz devices, such as filters, absorbers, and modulators.

Comprehensive Non-volatile Photo-programming Transistor Memory via a Dual-Functional Perovskite-Based Floating Gate

Photonic transistor memory has received increasing attention as next-generation optoelectronic devices for light fidelity (Li-Fi) application due to the attractive advantages of ultra-speed, high security, and low power consumption. However, most transistor-type photonic memories developed to date still rely on electrical bias for operation, imposing certain limits on data transmission efficiency and energy consumption. In this study, the dual manipulation of "photo-writing" and "photo-erasing" of a novel photonic transistor memory is successfully realized by cleverly utilizing the complementary light absorption between the photoactive material, n-type BPE-PTCDI, in the active channel and the hybrid floating gate, CH₃NH₃PbBr₃/poly(2-vinylpyridine). The fabricated device not only can be operated under the full spectrum but also shows stable switching cycles of photo-writing (PW)-reading (R)-photo-erasing (PE)-reading (R) (PW-R-PE-R) with a high memory ratio of ∼10⁴, and the memory characteristics possess a stable long-term retention of >10⁴ s. Notably, photo-erasing only requires 1 s light illumination. Due to the fully optical functionality, the rigid gate electrode is removed and a novel two-terminal flexible photonic memory is fabricated. The device not only exhibits stable electrical performance after 1000 bending cycles but also manifests a multilevel functional behavior, demonstrating a promising potential for the future development of photoactive electronic devices.

Acoustic frequency combs using gas bubble cluster oscillations in liquids: a proof of concept

We propose a new approach to the generation of acoustic frequency combs (AFC)-signals with spectra containing equidistant coherent peaks. AFCs are essential for a number of sensing and measurement applications, where the established technology of optical frequency combs suffers from fundamental physical limitations. Our proof-of-principle experiments demonstrate that nonlinear oscillations of a gas bubble cluster in water insonated by a low-pressure single-frequency ultrasound wave produce signals with spectra consisting of equally spaced peaks originating from the interaction of the driving ultrasound wave with the response of the bubble cluster at its natural frequency. The so-generated AFC posses essential characteristics of optical frequency combs and thus, similar to their optical counterparts, can be used to measure various physical, chemical and biological quantities.

Terahertz Frequency Combs Exploiting an On-Chip, Solution-Processed, Graphene-Quantum Cascade Laser Coupled-Cavity

The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain spectra, and with a full compensation of the group velocity dispersion, at terahertz (THz) frequencies, is key for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCSs) in the far-infrared. In THz QCLs four-wave mixing, driven by intrinsic third-order susceptibility of the intersubband gain medium, self-locks the optical modes in phase, allowing stable comb operation, albeit over a restricted dynamic range (∼20% of the laser operational range). Here, we engineer miniaturized THz FCSs, comprising a heterogeneous THz QCL, integrated with a tightly coupled, on-chip, solution-processed, graphene saturable-absorber reflector that preserves phase-coherence between lasing modes, even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes, through 55% of the laser operational range. We also achieve stable injection-locking, paving the way to a number of key applications, including high-precision tunable broadband-spectroscopy and quantum-metrology.

Graphene H-Waveguide for Terahertz Lasing Applications: Electromagnetic Quasi-Linear Theory

A novel graphene H-waveguide is proposed for active terahertz components. A graphene film illuminated by strong pumping light shorts the parallel conductor plates. The terahertz modes propagating along this film are amplified at certain conditions. A rigorous electromagnetic (EM) quasi-linear method of analytical calculations of TEy and TMy eigenmodes is used in this paper to select these conditions. Among them is the use of bound TEy modes interacting with graphene plasmons at frequencies of negative graphene resistance, minimizing conductor loss associated with parallel plates, and excluding the current-crowding effect from the waveguide design. The limitations of the used theory are considered, and the applications of this waveguide are proposed.

Investigation of Phonon Scattering on the Tunable Mechanisms of Terahertz Graphene Metamaterials

The influences of different kinds of phonon scatterings (i.e., acoustic (AC) phonon, impurity, and longitudinal optical (LO) phonon scatterings) on the tunable propagation properties of graphene metamaterials structures have been investigated, also including the effects of graphene pattern structures, Fermi levels, and operation frequencies. The results manifested that, at room temperature, AC phonon scattering dominated, while with the increase in temperature, the LO phonon scattering increased significantly and played a dominate role if temperature goes beyond 600 K. Due to the phonon scatterings, the resonant properties of the graphene metamaterial structure indicated an optimum value (about 0.5-0.8 eV) with the increase in Fermi level, which were different from the existing results. The results are very helpful to understand the tunable mechanisms of graphene functional devices, sensors, modulators, and antennas.